The biphase L code is sometimes known as the "Manchester" code and transforms a sequence of bits which are made available for transmission at a rate 1/T into a transmission signal which comprises a succession of non-overlapping two-level signals each of which lasts for one Baud period T and each of which has a transition in the middle of each Baud period in a direction which depends on the value of the bit to be encoded. This transmission signal is obtained by combining successive bits of the sequence to be transmitted with the clock signal driving said sequence by means of an exclusive-OR type of logic gate.
The modified biphase code differs from the biphase L code in that the clock signal is phase shifted by .pi./2.
The biphase L and the modified biphase codes may be considered as being double side band modulations of the following two Walsh functions in quadrature: EQU sign (sin(2.pi.t/T)); sign (cos(2.pi.t/T))
and they may be combined by quadrature modulation to define a code which is referred to hereinafter as Wacx code and which gives a line signal z' (t) which may be expressed by the following equation in which a linear scale factor is omitted: ##EQU1## where a.sub.K and b.sub.K are two synchronous sequences of binary data to be transmitted providing values of .+-.1 at a rate 1/T, and where d(t-KT) is a time function having the value unity for the K-th Baud interval and nill otherwise. This Wacx code has the advantage of doubling the data rate compared with the biphase and modified biphase codes, without doubling the occupied bandwidth. A detailed study of this code is to be found in the Applicants' French patent application No. 82 02227 filed Feb. 11, 1982.
Preferred embodiments of the present invention provide synchronous data transmission equipment using the Wacx code, which equipment is simple and cheap, and suitable for use in providing a data transmission link over existing line plant between a telephone exchange and subscriber equipment.